Working with an in vitro model for invasion of red cells by malaria merozoite, this study has demonstrated that human red cells lacking the Duffy a and b antigens are resistant to invasion. This corresponds well with epidemiologic studies which show that approximately 70% of West African and American blacks are resistant to Vivax malaria. A similar proportion of these blacks are Duffy a and b negative whereas the Duffy negative genotype is extremely rare in other racial groups. The close association between the Duffy negative genotype and resistance to Vivax malaria has also been confirmed by retrospective analysis of 11 volunteers experimentally inoculated with P. Vivas. A larger study in humans is planned; this will involve correlation of Duffy phenotypes of U.S. servicemen who acquired malaria in South-East Asia and comparison with the frequencies of Duffy phenotypes in a control population. BIBLIOGRAPHIC REFERENCES: Miller, L.H., Mason, S.J., Clyde, D.F., and McGinniss, M.H.: The resistance factor to Plasmodium vivax in blacks. N. Eng. J. Med. 295: 302-304, 1976. Miller, L.H., Haynes, J.D., McAuliffe, F.M., Shiroishi, T., Durocher, J.R., and McGinniss, M.H.: Evidence for differences in erythrocyte surface receptors for the malarial parasites, Plasmodium falciparum and Plasmodium knowlesi. J. Exp. Med. 146: 277-281, 1977.